Circuit Description
The evaporative emission (EVAP) system is used to store fuel vapors as to reduce
the amount of unburned fuel from escaping into the atmosphere. The EVAP system consists
of the EVAP canister, fuel tank pressure (FTP) sensor, EVAP lines and hoses, EVAP
purge solenoid - normally closed, EVAP service port, EVAP vent solenoid -
normally open, and the fuel tank. The engine control module (ECM) monitors the EVAP
system for circuit faults in the EVAP purge solenoid, EVAP vent solenoid, and FTP
sensor circuits. The ECM also monitors the EVAP system for leaks, between the EVAP
vent and EVAP purge solenoid. When certain conditions have been met, the ECM will
perform an EVAP diagnostic test at idle with no vehicle speed, which will require
a 5 consecutive phase, A, B, C, D, and E test. DTC P0446 will set if
too much vacuum is present with the EVAP vent solenoid commanded OFF during Phase A
or Phase C of the EVAP diagnostic system test indicating a blocked EVAP vent
system.
Conditions for Setting the DTC
DTC P0446 will set if fuel tank pressure indicates less than 1.3 volts
less than -11.21 mm Hg (-6.00 in H2O) during
Phase A of the EVAP diagnostic system test or less than 2.02 volts less
than -4.5 mm Hg (-2.41 in H2O) during Phase C
of the EVAP diagnostic test indicating a blocked vent system when:
| • | The condition exists for 15 seconds for Phase A or condition
is present by the end of Phase C. |
| • | The EVAP purge solenoid is commanded to a certain duty cycle. |
| • | The EVAP vent solenoid is commanded OFF. |
| • | The vehicle speed is 0 mph. |
| • | The engine load is less than 35 percent. |
| • | The MAP is less than 40 kPa. |
| • | The battery voltage is greater than 11 volts. |
| • | The startup ECT is between 8-50°C (46-167°F). |
| • | The IAT is between 8-50°C (46-122°F). |
| • | The long term fuel trim cells are learned. If not learned, the engine
must run longer than 16 minutes and 45 seconds. |
| • | The fuel level is between 6-48 L (1.6-12.7 gallons). |
| • | No ECT, EVAP purge solenoid, EVAP vent solenoid, fuel tank pressure, idle
speed, HO2S, throttle actuator, TP sensor, or vehicle speed DTCs have been set. |
DTC P0446 diagnostic runs once during Phase A and Phase C
of the EVAP diagnostic test once per ignition cycle.
If an EVAP diagnostic test is aborted due to a changing enable criteria, the
ECM will attempt to rerun the EVAP diagnostic test up to 3 times per ignition
cycle.
DTC P0446 is a type B DTC.
Diagnostic Aids
Important:
| • | If any EVAP system DTC is set, EVAP special functions on the scan tool
will not be allowed. Clear the DTCs before performing these tests. |
| • | The ECM uses the fuel tank pressure sensor to detect the vapor in the
fuel tank during all parts of the EVAP diagnostic system test. Make sure the fuel
tank pressure sensor is not skewed by verifying FUEL TANK PRESS. SENSOR on the scan
tool is between 2.35-2.85 volts with the ignition ON and the engine OFF
and the fuel cap removed. |
Refer to EVAP System Description at the end of this section for a complete EVAP
diagnostic test description.
Possible causes of a blocked EVAP vent system:
| • | EVAP vent solenoid blocked or stuck valve |
| • | EVAP vent hose from EVAP canister to EVAP vent line or EVAP hose at EVAP
vent box kinked or pinched |
| • | EVAP canister restricted |
